I have an shaft/crank that will be turned by the user. I want to precisely and dynamically control the torque required to do so.

For simplicity we can think of it as a exercise machine (so the torque will be pretty high, depending on the length of the crank arm). User rotates a crank, computer controls how hard it is to do so. Like a hand powered crane with a simulated load.

I'm looking for advice on how to do that.

Simplest solution, where the rope with a weight at the end is attached to the shaft lacks gradual control.

Solutions I was thinking about:

  1. Disc brake, even from a bicycle and a servo controlling the clamping force. - I think this will be very hard to precisely control (in terms of the torque needed to turn).
  2. Backdrivable electric motor with gearing - But for high torque that would require a lot of gearing / a powerful motor, plus that would mean the motor is constantly being backdriven, which would probably increase the risk of damaging it (or the controller).
  3. Some combination of weights and motors where most of the torque is provided by the weight, but motor is there to adjust it.

I need your opinions, suggestions and ideas.

  • $\begingroup$ A "backdrivable electric motor" is called a "generator". You will need a variable resistance electrical load (probably with a heat sink or even water cooling) to dissipate the electrical energy as heat. $\endgroup$
    – alephzero
    Commented Nov 15, 2020 at 0:45

1 Answer 1


Like you said there are many ways to do it. Computer controlled and usually expensive.


These are the tools built exactly for this type of task. Obviously they are amognst the most expensive for the job. The most difficult part would be to determine the specs (what is the maximum torque, what is the control resolution, is it computer controlled etc ). You can see what a dynamometer is at various links (e.g. 1.

Electrical system with inverter and motor

What you can do is connect your shaft to a generator/motor, and control though an inverter the torque. Basically, the motor is free to turn, but as you increase the electric current passing through the coils it is harder and harder to move the rotor.

Similar idea but reversed is the following. you could connect the shaft to a generator and connect it to a inverter which is connected to the grid. You could control how much power is absorbed (and therefore the torque), and you could put that energy to the grid (so you would have a human generator).

In both cases you'd need a encoder for the shaft rpm.


  • torque should be controlled quite nicely (barring friction and cogging torque)


  • expensive
  • requires programming knowledge

Mechanical brakes

There are many variations here. The main problem is controlling the torque accurately. Basically you will be relying on the coefficient of friction, which can change depending on wear, and other conditions e.g. humidity.

Wear might not seem relevant however, because you will be applying it constantly, it will become an issue sooner rather than latter.

  • Drum brake

you could use the computer to actuate the brake and control whether the applied torque.

enter image description here

  • Disc Brake:

this is a similar to the drum brake.

  • Belt Friction

you can use a belt and apply tension on the belt in order to increase the friction force between belt and trum. If the drum is connected to the shaft it connect the torque on the shaft. enter image description here

  • Many other variations...

Constant torque and constant force in mechanical systems

One main problem when using friction to mechanically control torque is that the torque might exhibit large variations. In order to mitigate that it is important to apply a constant force on the friction surface. One (good) way to do that is insert a spring between the actuator and the controlled surface. Then the displacement of the actuator will control the displacement of the spring and indirectly the force. If you have a sufficiently long travel for the spring then you can have a very good control on the force. A nice (but rather old) description is at this link.

  • $\begingroup$ Drum brakes and disc brakes are similar as in they use friction but the effectiveness of the disc brake is much higher. $\endgroup$
    – Solar Mike
    Commented Nov 15, 2020 at 7:14

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